1. Field of the Invention
The invention relates to thermal writing devices for recording apparatus using heat-sensitive paper, comprising:
a nib including a writing means and a resistor having an electric resistance substantially dependent on temperature, and
an electric supply and control circuit which supplies the resistor and controls its temperature so as to heat the writing means.
2. Prior art
In devices of the aforementioned kind, the heat-sensitive paper is moved under the writing means, which is heated by the resistor. The heat of the writing means brings about a chemical reaction which leaves a visible track on the card.
The intensity and width of the track depend on the shape of the writing means, the pressure which it exerts on the paper, the speed with which it moves along the paper, and the temperature of the actural writing means.
However, in thermal writing devices of the aforementioned kind, the problem arises of regulating the temperature of the writing means.
According to a known method, the writing means comprises a stylus made of heat-conducting material in an electrically insulating holder or sleeve, round which the heating resistor is coiled. The resistor is supplied by the supply and control circuit, which regulates the supply current in dependence on the resistance of the resistor, so as to keep the temperature substantially equal to a predeterminated reference value.
However, the aforementioned thermal writing devices have some disadvantages, the main disadvantage being that, owing to the high resistance and thermal capacity introduced by the sleeve between the resistor and the writing means, the temperature of the writing means does not remain at the reference value but undergoes substantial oscillations around it. Such temperature variations may have an unacceptable effect on the uniformity and regularity of the visible track.
The aforementioned disadvantage is particularly serious when the heat-sensitive paper used is in two or more colours, when the colour of the track depends closely on the temperature of the writing means.
A second disadvantage is that, because of the thermal characteristics of the sleeve, the writing means is not excessively urged towards thermal equilibrium. As a result, recording apparatus using the aforementioned thermal writing devices is unsuited for recording rapidly-varying phenomena.
An object of the invention, therefore, is to construct a thermal writing device of the aforementioned kind which obviates the aforementioned disadvantages and is also rugged and wear resistant, economic to manufacture and may be used with advantage, even in multi-track recording apparatus.